1. Technical Field
This invention relates to the field of lithography in semiconductor fabrication. More specifically, the invention relates to a compound with a polysilsesquioxane (PSQ) backbone, a method to synthesize such a PSQ compound, a polymer resin including such a PSQ compound, a photoresist composition incorporating such a polymer resin, and a method for performing multilayer lithography using such a photoresist composition.
2. Background Art
The need to remain cost and performance competitive in the production of semiconductor devices has caused continually increasing device density in integrated circuits. To facilitate the increase in device density, new technologies are constantly needed to allow the feature size of these semiconductor devices to be reduced. Adjusting or reformulating photoresist compositions constitutes one attempt to provide high resolution capability (approximately 300 nanometer (nm) resolution or less) and wider process latitude. However, as the desired feature size decreases, the resolution capability of many current resists, even those that are reformulated, is not sufficient to yield the smaller features. The need to achieve less than 300 nm resolution has prompted a push toward increasing numerical aperture (NA) in exposure tools from 0.5 to as high as 0.7 and forming thinner photo resist films on substrates. The higher NA allows for improved resolution of smaller feature sizes, however, the higher NA also reduces the depth of focus of aerial images projected onto the photoresist film. Since the depth of focus is more shallow, the thickness of the photoresist film becomes a factor in properly exposing the photoresist. Thus, thinner photoresist films may be required for proper exposure at high resolution, but often do not yield acceptable overall performance, especially when considering etch requirements for the underlying substrate.
As the photoresist film is thinned to account for the higher NA, the photoresist becomes less suitable as an etch mask against later processing of the underlying semiconductor substrate. For example, since the resist film is thin, variation in thickness becomes more significant than it does in thicker resist films and may introduce defects into subsequent devices formed on the substrate. Also, micro-channels often form in the upper portions of the resist layer during transfer of the resist image to the substrate by etching. When the resist is thin, the micro-channels may extend to the underlying substrate, rendering the photoresist less effective as a mask. Because higher resolution requires higher numerical aperture, and higher numerical aperture in turn requires thinner resists, it is very difficult to develop a total lithography/etch process using current single layer resists.
In addition, the process latitude of many current resists is not sufficient to consistently produce the smaller desired features within specified tolerances. Variations, even though small, are inherent in the lithographic processes that create the resist images used to form devices on semiconductor substrates. Some of the process parameters subject to variation include bake time and temperature, exposure time and source output, aerial image focus, and develop time and temperature. The process latitude of a resist is an indication of how wide such variations can be without resulting in a change in the resolution and/or image profile (i.e., size and/or shape of a resist image). That is, if the process latitude is sufficiently wide, then a process parameter may vary, but the variance will not produce a change in the resist image incompatible with specified tolerances.
One approach that enables the use of higher NA exposure tools as well as a thinner photoresist film is multilayer resist processing. One type of multilayer resist processing uses a bilayer (two layer) imaging scheme by first casting a highly energy absorbing underlayer on the semiconductor substrate then casting a thin, silicon-containing imaging layer (photoresist film) on top of the underlayer. Next, selected portions of the silicon-containing layer are exposed and developed to remove the unexposed portions of a negative photoresist film or the exposed portions of a positive photoresist film. Generally, the underlayer is highly absorbing at the imaging wavelength and is compatible with the imaging layer. Interactions to be considered include adhesion between the two layers, intermixing, and contamination of the imaging layer by the components of the underlayer. Also, the refractive index of the underlayer is matched to the refractive index of the silicon-containing resist layer to avoid degrading the resolution capability of the silicon-containing resist.
Conventional underlayers include diazonapthoquinone (DNQ)/novolak resist material or novolak resin cast on the semiconductor substrate. For the imaging layer, resists containing a wide variety of silicon-containing polymers have been used, including silsesquioxane, silicon-containing acrylics, silanes, etc. Among the several possible silicon-containing polymers, aqueous base-soluble silsesquioxane polymers, such as poly(p-hydroxybenzylsilsesquioxane) (PHBS), have emerged as the most promising candidates for silicon-containing polymers in bilayer resist systems. Unfortunately, although it is promising, PHBS suffers from several short comings.
First, the current methods for synthesis of PHBS and related aqueous base-soluble silsesquioxane polymers involve using BBr.sub.3 to demethylate the intermediate poly(p-methoxybenzylsilsesquioxane) (PMBS) that forms when synthesizing PHBS. The use of BBr.sub.3 is difficult to control and produces unwanted byproducts, chief among which is HBr, an acidic resist contaminant. Since the HBr contaminates the polymer resin, a separate process must be completed to remove HBr prior to preparation of the desired resist composition. Thus, there is a need to provide a silicon-containing polymer for a multilayer resist composition that is not contaminated with byproducts and that may be synthesized using a more controllable process.
In addition, each of the PHBS-based bilayer resists only produce high resolution (less than or equal to 300 nm) when developed with a relatively weak developer, typically, 0.14 normal (N) tetramethylammoniumhydroxide (TMAH). Unfortunately, the 0.14 N developer is not an industrial standard developer. Accordingly, 0.14 N TMAH often must be purchased specially for the bilayer imaging process at a higher cost than standard developers with the requirement of additional storage and handling facilities. Accordingly, there is a need to provide a silicon-containing bilayer resist system that is compatible with industrial standard developers, including 0.26 N TMAH.
An additional limitation of current silsesquioxane-containing photoresists is that they have a low glass transition temperature. Thus, the current silsesquioxane-containing resists can not be processed at very high temperatures after exposure as may be desired to complete crosslinking reactions in a negative photoresist system in a timely and sufficient manner. This is a further limitation on the ability of current silicon-containing photoresists to generate high resolution patterns in a commercially viable process.
Thus, there is a need for improved silsesquioxane polymers useful in photoresist compositions, improved methods of making silsesquioxane polymers for use in photolithography applications, and improved silsesquioxane polymer-containing photoresist compositions.